Choosing the right material for milling holder adapter

There is an adapter here in our workshop for milling holder BT shanks (that converts for example BT-60 to BT-40). We have decided to build a new one according to the sample. The surface hardness is about 60 HRC, we tested the material for analysis and it was 1.8519 (31CrMov9) (nitriding steel). I was thinking whether it'd be possible to replace the material with a more widespread grade (such as 42CrMo4) as the adapter is required to be nitrided. Also, the work-piece is exposed to water-based liquids. Should it be taken as an design consideration for choosing the material and the surface hardening method? Does the more %C in 42CrMo4 (and more reacting tendency with Cr) affect the anti-oxidation properties of the material? If that's so, what about selecting a 420 steel? Is it possible to nitride it?

The sample adapter you evaluated was probably engineered to be durable in the widest range of situations. I agree that for your specific application, you may well be able to reduce the specifications and cost. Sleeve adapters must be accurate and stable, they probably do not need to be very hard. I have read carefully and tried to find answers to all the technical questions you have asked here, and they are all very good questions. But they all hinge on the degree of compromise acceptable in your situation.

By asking these questions you have crystallised and encapsulated your fears and doubts over the wisdom of compromise. The truth is that you can probably get away with compromise if you make an adapter quickly “for evaluation” at the lowest possible cost. All is not predictable, experimentation is necessary to advance.

Your questions demonstrate that you understand well the situation and the lack of certainty. The questions are actually more psychological than metallurgical. It is probably time now to make an existentialist decision, that is, without the secure information you and/or a bureaucracy would demand. If you cannot make that decision then it probably does not matter either way, so spread the blame by passing your proposals to a superior, or just toss a coin and get on with it.

The cost of achieving perfection is infinite. The answer to all your questions should be;
1. Either buy another adapter that works, like the sample you now have;
2. Or develop a flexibility by making a few different adapters for evaluation over time using low cost available materials.

If you ask the metallurgists who make the nitriding steels they will strongly recommend the best available nitriding steel, at their price. The question comes down to the cost of machining a trial unit. I would start by making an adapter from an available steel such as 420 to verify and evaluate my machining processes. I would not nitride evaluation trial units.